144 
SCIENCE. 
tubercles, and an Echinid poriferous zone. In the same 
way in the Diadematidae, the large primary internambulac- 
ral tubercles are Cidaridian features, while the structure of 
the ambulacral tubercles is Hemicidaridian. The existence 
of two kinds of spines is another Cidaridian feature, 
while the apical and actinal systems have become modi- 
fied in the same direction as that of the Echinidae. The 
more recent the genus, the greater is the difficulty of 
tracing in a direct manner the origin of any one structural 
feature, owing to the difficulty of disassociating structural 
elements characteristic of genera which may be derived 
from totally different sources. This is particularly the case 
with genera havi ng a great geological age. Manv of them, 
especially among the Spatangoids, show affinities with gen- 
era following them in time, to be explained at present only 
on the supposition that, when a structural feature has once 
made its appearance, it may reappear subsequently, appa- 
rently as a new creation, while in reality it is only its pecu- 
liar combination with structural features with which it had 
not before been associated (a new genus), which conceals 
in that instance the fact of its previous existence. A care- 
ful analysis, not only of the genera of the order, but some- 
times ot otherorders which have preceded this combination 
in time, may often reveal the elements from which have 
been produced apparently unintelligible modifications. 
There is, however, not one of the simple structural fea- 
tures in the lew types of the Triassio and Liassic Echini 
from which we can so easily trace the origin of the structural 
features of all the subsequent Echinid genera, which is not 
also itself continued to the present day in some generic 
type of the present epoch, fully as well characterized as it 
was at the beginning. In fact, the very existence to-day of 
these early structural features seem to be as positive a proof 
of the unbroken systematic affinity between the Echini of 
our seas and those of the Trias, as the uninterrupted exist- 
ence of the genus Pygaster or Cidaris from the Trias down 
to the present epoch, or of the connection of many of the 
genera of the Chalk with those of our epoch (Salenia, 
Cyphosoma, Psammechinus, etc.). 
Passing to the Clypeastridae, we find there as among the 
Desmosticha that the earliest type, Pygaster, has existed 
from the Trias to the present time ; and that, while we can 
readily reconstruct, on embryoiogical grounds, the modifi- 
cations the earliest Desmosticha-like Echini should under- 
go in order to assume the structural features of Pygaster, 
yet the earl)' periods in which the precursors of the Echino- 
conidse and Clypeastridae are found have thus far not pro- 
duced the genera in which these modifications actually 
take place. But, starting from Pygaster, we naturally pass 
to Holectypus, to Discoidea, to Conoclypus, on the one 
side, while on the other, from Holectypus to Echinocyamus, 
Sismondia, Fibularia, and Mortonia, we have the natural 
sequence of the characters of the existing Echinanthidae, 
Laganidae, and Scutellidae, the greater number of which are 
characteristic of the present epoch. If we were to take in 
turn the changes undergone in the arrangement of the plates 
of the test, as we pass from Pygaster to Holectypus, to 
Echinocyamus, and Echinanthidae, we should have in the 
genera which follow each other in the paleontological 
record an unbroken series showing exactly what these 
modifications have been. In the same way, the modifica- 
tions of the abactinal and anal systems, and those of the 
poriferous zone, can equally well be followed to Echino- 
cyamus, and thence to the Clypeastridae; while a similar 
sequence in th ■ modifications of these structural features 
can be followed from Mortonia to the Scutellidae of the 
present period. 
Passing finally to the Petalosticha, we find no difficulty 
in tracing theoretically the modifications which our early 
Echinoconidae of the Lias should primarily undergo previous 
to the appearance of Galeropygus. The similarity of the 
early Cassiduloid and Echinoneoid types points to the 
same systematic affinity, and perhaps even to a direct and 
not very distant relationshiD with the Pahechimdae. For if 
we analize the Echinothuriae of the present day, we find in 
genera like Phormosoma many structural features, such as 
the shape of thetest the character of the spines, the structure 
of the apical system, that of the poriferous zone, indicative 
of possible modifications in the direction of Pygaster or of 
Galeropygus, which have as yet not been taken into 
account. 
Adopting for the Petalosticha the same method of trac- 
ing the modifications of single structural features in their 
paleontological succession, we trace the comparatively 
little modified paleontological history of the Echinoneidie 
of the present day from the Pyrina oi the lower Jura. This, 
in its turn, has been preceded by Ilyboclypus and Galero- 
pygus, while the Echinolampada; oi the present day date 
back, with but trifling modifications, to the Echinobrissus 
of the Lias, itself preceded by Clypeus ; and they have 
been subject only to slight generic changes since that time, 
Echinobrissus being still extant, while such closely allied 
genera as Catopygus and Cassidulus of the earlier Creta- 
ceous are still represented at the present day ; the modifi- 
cations taking place in the actinal system, in the ambulacal 
zones of the Echinoconidae and of the Echinolampadae 
showing the closest possible systematic affinity in these 
families. Starting again from Ilyboclypus, with its elongate 
apical system, we naturally pass to Collyrites and the 
strange Dysasteridse forms which, in their turn, are closely 
allied to the Iiolasteridae. r s From Holaster on the one side, 
and from Toxaster on the other, we find an unbroken 
sequence of structural characters uniting the successive 
genera of Holastoridse, such as Cardiaster, OfTaster, Stenonia, 
Ananchytes, and Asterostoma, with Paleopneustes, Homo- 
lampas and the Pourtalesiae of the present day, while from 
the genera of the Toxasteridae we naturally pass to the 
cretaceous Hemiaster ; in the genus and the subsequent 
Micraster we find all the elements necessary for the modi- 
fications which appear in the Spantanginae from the time of 
the Chalk to the present day. Tnese modifications result 
in genera in which we trace the development of the fascioles, 
of the actinal, anal, and abactinal plastrons, of the beak, 
the formation of the pelaloid ambulacra, first flush with the 
test, and little by little changed into marsupial pouches, 
the growth of the anterior groove and the manifold modifi- 
cations of the ambulacral system in Spatangus Agassizia, 
and Echinocardium, often recalling in some of its features 
structural characters of families which have preceded this 
in time. 
Apparently in striking contrast with the Echini of the 
secondary period and those which have succeeded them 
stand the Paleozic Echini ; but when we have examined the 
embryology of Echini, we shall be better prepared to under- 
stand their structure and the affinities of the Palaechinidae 
with the Echini of the present day and their immediate pre- 
decessors. 
Taking up now the embryoiogical development of the 
several families which will form the basis of our compari- 
sons, beginning with the Cidaridse, we find that in the 
earliest stages they very soon assume the characters of the 
adult, the changes being limited to the development of the 
abactinal system, the increase in number of the coronal 
plates, and the modifications of the proportionally gigantic 
primary radioles. 
In the Diadematidae the changes undergone by the young 
are limited to the gradual transformation of the embryonic 
spines into those which characterize the family, to the 
changes of the vertical row of pores in the ambulacral 
area into arcs of three or four pairs of pores, and to the 
specialization of the actinal and abactinal systems. 
In the Arbaciadm the young stages are remarkable for the 
prominent sculplure of the test, for the flattened spines, for 
their simple poriferous zone, for their actinal system, and for 
their genital ring. The anal plates appear before the geni- 
tal ring. 
In the Echinometradas the young thus far observed are 
characterized by the small number of their primary tuber- 
cles, the large size of the spines, the simple vertical row of 
pores, the closing of the anal ring by a single plate, and the 
turban-shaped outline of the test. Little by little, the test 
loses with increasing age this Cidaris-like character ; it re- 
minds us, from the increase in the number of its plates, 
more of Hemicidaris ; then, with their still greater increase, 
of the Pseudodiadematidre ; and, finally, of the Echinome- 
trada; proper. The spines, following pari passu the changes 
of the test, lose little by little their fantastic embryonic, or 
rather Cidaris-like appearance, and become more solid and 
shorter, till they finally assume the delicately fluted structure 
characteristic of the Echinometradae. The vertical porife- 
rous zone is first changed into a series of connected verti- 
cal arcs, which become disjointed, and form, with increas- 
